The same as 'datum if datum does not include
(unquoteexpr) or (unquote-splicingexpr). An
(unquoteexpr) form escapes from the quote, however,
and the result of the expr takes the place of the
(unquoteexpr) form in the quasiquote result. An
(unquote-splicingexpr) similarly escapes, but the
expr must produce a list, and its elements are spliced as
multiple values place of the (unquote-splicingexpr), which
must appear as the car or a quoted pair, as an element of a
quoted vector, or as an element of a quoted prefab structure;
in the case of a pair, if the cdr of the relevant quoted pair
is empty, then expr need not produce a list, and its result
is used directly in place of the quoted pair (in the same way that
append accepts a non-list final argument). In a quoted
hash table, an (unquoteexpr) or
(unquote-splicingexpr) expression escapes only in the
second element of an entry pair (i.e., the value), while entry keys
are always implicitly quoted. If unquote or
unquote-splicing appears within quasiquote in any
other way than as (unquoteexpr) or
(unquote-splicingexpr), a syntax error is reported.